1) Ham whip clamped to the stern rail, or bimini
framework. Higher is better.
2) Marine VHF
whip clamped to the stern rail (SWR won't be great, but it will work)
3) Vertical dipole made from coax (this is definitely the cheapest). Tie one end to the spreader with light line, and the other to a lifeline or something. Run the center feed horizontally for a bit to minimize feedline interaction.
4) Vertical coaxial dipole, made from coax. Just as cheap
as (3), and easier to rig. This will take some tuning for best performance. Strip off 1/4 wave of outer insulator, and work the shield braid back down the coax. I would tape over the exposed shield -- this will protect the bare wires from corrosion
, at least for a little while.